In most oil refining processes, uncalcined coke is produced from petroleum residual fractions. Many processes are known for producing uncalcined coke. Of the coking processes in commercial use, such as delayed coking, fluid coking and flexi-coking, delayed coking has emerged as the technology of choice due to its lower investment cost and its ability to provide comparable yields of products, but of higher quality.
A typical delayed coking process is a semi-continuous process in which heavy hydrocarbon feedstock is heated to cracking temperature using a heat source such as a coker furnace. The heated feedstock is then fed continuously to a coking drum, where it reacts in its contained heat to convert the feedstock to coke and cracked vapors. The cracked vapors, which are normally the primary objective of the coking process, are passed overhead to a coker fractionator, condensed and recovered as lower boiling hydrocarbon products. When the coke drum contents reach a predetermined level, the feedstock supply is switched to another drum, and the full drum is cooled and de-coked. The material removed from the drum is commonly referred to as “uncalcined” or “green” petroleum coke, or “uncalcined coke”.
This general delayed coking process is well known and many variations of the process exist. An exemplary version of the process is disclosed in U.S. Pat. No. 4,720,338.
The uncalcined coke recovered from the various coking processes may be subjected to a variety of additional process steps.
For example, uncalcined coke contains volatile combustible material (VCM) and water. Green coke that is considered suitable for use in the aluminum or steel industries or in the production of titanium oxide is typically calcined to remove VCM and associated moisture and to otherwise improve critical physical properties such as density and crystalline structure.
Rotary kilns are frequently used for calcining coke. When a rotary kiln is used, particulate green coke is fed into one end of an inclined rotating and generally cylindrical kiln having burners near the discharge end of the kiln to provide heat for transforming the uncalcined coke into calcined coke as the coke moves through the calciner. Other systems suitable for calcining coke include rotary hearth furnaces and shaft kilns.
Descriptions of coke calcining processes and rotary kiln designs can be found in U.S. Pat. Nos. 4,022,569 and 4,053,365. The exhaust from rotary kiln calciners, and other types of calciners, contains entrained coke particles and the volatile components driven from the uncalcined coke. This exhaust is known as calciner effluent gas or “offgas.”
Environmental regulations throughout the world require that the entrained coke particles and volatile components in the calciner effluent gas be captured and disposed of to prevent releasing these materials into the environment. The calciner effluent gas is often disposed of by mixing the effluent gas with air and burning the mixture in an incinerator. The energy released during burning the offgas may be captured by heating water to produce steam for use in heat transfer processes or for electric power generation.
It is also known that coke may be burned for its high heat value as disclosed in U.S. Pat. No. 4,602,575. U.S. Pat. No. 6,570,049 discloses an incineration system for burning a variety of materials, including petroleum coke.